IPA: /ˌʌnɪkˈsɜːrsaɪzd/
KK: /ʌnˈɛksərˌsaɪzd/
Not having been used or practiced, especially in relation to physical activity or skills.
After a long winter, my muscles felt unexercised and weak.
Unexercised → It is formed from "un-" (meaning not) and "exercised" (from Latin "exercitare", meaning to train or practice). The word "unexercised" means not having been trained or practiced.
Think of 'not' ('un-') having been 'trained' ('exercised') — that's why unexercised means not trained.